移动端下拉子菜单如何实现展开时向下推送其他列表项？

问题核心原因 #

你的移动端子菜单之所以会覆盖其他顶级项，是因为全局样式里给.sub-menu-content设置了position: absolute，绝对定位的元素会脱离正常文档流，不占用空间，自然无法推挤下方元素。而你测试的手风琴组件用的是静态/相对定位，属于文档流内元素，所以能正常推送。

具体修改步骤 #

1. 修正移动端子菜单定位 #

在移动端媒体查询（@media (max-width: 1025px)）中，给.sub-menu-content添加position: static，覆盖全局的绝对定位，同时调整间距：

.sub-menu-content {
  position: static; /* 关键：让子菜单回到文档流 */
  margin-top: 0; /* 覆盖全局的负margin */
  padding:0;
  background-color: white;
  display: none;
  overflow: auto!important;
  box-shadow:none;
  border-radius:0;
  width:100%;
}

2. 取消父项固定高度 #

移动端的.nav-item设置了固定height: 60px，子菜单展开后会被截断，改成自适应高度：

.nav-item {
  border-top: none;
  margin-left: 0;
  box-sizing: border-box;
  border-bottom: 1px solid #b5b5b5;
  width: 100%;
  text-align: left;
  line-height: 60px; /* 保留行高保证顶级项垂直居中 */
  height: auto; /* 关键：自适应高度 */
  display: block;
}

3. 优化JS函数（支持多子菜单） #

原来的函数依赖固定ID，改成基于点击元素的相对查找，避免ID冲突：

function subOpen(el) {
  // 找到当前点击元素的兄弟子菜单容器
  const subMenu = el.parentNode.querySelector('.sub-menu-content');
  subMenu.classList.toggle("show");
}

同时修改HTML中的点击事件：

<a onclick="subOpen(this)" class="sub-menu-link">link 1...

修改后的完整代码 #

HTML #

<div class="header">
<div class="header-cont">
<div class="header-logo">
    <a aria-hidden="true" href="/">
      LOGO
</a>
</div>
<nav class="header-nav">
    <input type="checkbox" id="check">
      <label for="check" class="checkbtn">
          <div class="menu-btn">
    <div class="menu-btn__burger"></div>
  </div>
      </label>
      
<ul class="header-menu">
        <li class="nav-item">
            <div class="sub-menu">
                <a onclick="subOpen(this)" class="sub-menu-link">link 1<svg class="drop-chevron" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M233.4 406.6c12.5 12.5 32.8 12.5 45.3 0l192-192c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0L256 338.7 86.6 169.4c-12.5-12.5-32.8-12.5-45.3 0s-12.5 32.8 0 45.3l192 192z"/></svg></a>
               <div class="sub-menu-content">
        <ul>
    <li class="sub-item"><a href="#">sub link 1</a></li>
    <li class="sub-item"><a href="#">sub link 2</a></li>
    <li class="sub-item"><a href="#">sub link 3</a></li>
    <li class="sub-item"><a href="#">sub link 4</a></li>
    </ul>
    </div>
        </div>
        </li>
        <li class="nav-item"><a href="#" class="nav-link">link 2</a></li>
        <li class="nav-item"><a href="#" class="nav-link">link 3</a></li>
        <li class="nav-item"><a href="#" class="nav-link">link 4</a></li>
      </ul>
    </nav>
</div>
</div>

CSS #

body {
    margin: 0;
    font-family: -apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji;
    font-size: 1rem;
    font-weight: 400;
    line-height: 1.5;
    color: #333;
    background-color: #fff;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
  }
  a {
    text-decoration:none;
}
.header-cont,
.nav-link {
    align-items: center;
}
.header-nav,
.nav-item {
    height: 60px;
}
.header {
    background: #fff;
    box-shadow: 0 0 10px -3px rgb(0 0 0 / 50%);
}
.header-cont {
    display: flex;
    max-width: 1350px;
    margin: auto;
    justify-content: space-between;
    padding: 0 1rem 0 1rem;
    box-sizing: border-box;
}
.nav-link,
.sub-menu-link {
    padding: 0 1rem 0 1rem;
    font-weight: 600;
    color: #0f0f0f !important;
    cursor:pointer;
}
.header-menu li:last-child {
    margin-bottom: 0;
    padding-bottom: 0;
}
.only-mobile {
    display: none !important;
}
.nav-item,
.nav-link {
    display: inline-block;
}
.nav-item {
    line-height: 60px;
}
.drop-chevron {
    margin-left: 10px;
    height: 13px;
    fill: #0f0f0f;
}
.nav-item:hover svg {
    fill: #00007a !important;
}
.nav-item:hover {
    background-color: #00007a0d;
    transition: background-color 0.3s;
}
.nav-link:hover,
.sub-menu-link:hover {
    color: #00007a !important;
    transition: 0.3s;
    text-decoration: none !important;
}
.sub-menu {
    position: relative !important;
}
.sub-menu-link {
    display: inline-block !important;
    text-decoration: none !important;
}
#check,
.checkbtn {
    display: none;
}
.sub-menu-content {
    display: none;
    margin-top: -0.1rem;
    background-color: #fff !important;
    box-shadow: 0 6px 14px -1px rgb(0 5 20 / 15%);
    border-radius: 3px;
    overflow: hidden;
    position: absolute;
    width: 200px;
    z-index: 1000;
}
.sub-menu-content ul{
    list-style-type: none;
    line-height: 30px;
}
.sub-item {
    width: 200px;
    margin-left: -0.5rem;
}
.sub-menu-content li:last-child {
    border-bottom: 1px solid transparent !important;
    padding-bottom: 0.1rem !important;
    margin-bottom: -0.2rem;
}
.sub-menu-content a {
    color: #0f0f0f;
    width: 100%;
    padding: 0.8rem;
    margin-left: -2rem;
    text-decoration: none;
    display: block;
    text-align: left;
    border-left: solid 4px transparent;
}
.sub-menu-content a:hover {
    text-decoration: none;
    border-left: 4px solid #ff9f1c;
    background-color: #00007a0d;
    color: #00007a !important;
}
.checkbtn {
    font-size: 20px;
    color: #00007a;
    float: right;
    line-height: 60px;
    margin-right: 1rem;
    cursor: pointer;
}
@media (max-width: 1025px) {
    selector .header-logo {
        margin-top: 0.1rem;
        padding-top: 0;
    }
    .header-cont {
        justify-content: space-between;
    }
    .only-mobile,
    .sub-menu-link {
        display: block !important;
    }
    .checkbtn,
    .nav-link,
    .sub-menu {
        display: block;
    }
    .drop-chevron {
        display: inline-flex;
       position:absolute;
       right:17px;
       top:22px;
        
    }
    .sub-menu-content {
  position: static; /* 新增：回到文档流 */
  margin-top: 0; /* 覆盖全局负margin */
  padding:0;
  background-color: white;
  display: none;
  overflow: auto!important;
  box-shadow:none;
  border-radius:0;
  width:100%;
    }
    .sub-item {
    border-top:none;
    margin-left:-1rem;
    margin-top:0rem;
    margin-bottom:0px;
    box-sizing: border-box;
    line-height: 3rem;
    border-bottom: solid 1px #B5B5B5;
    width:100%;
}
.sub-menu-content li:last-child {
    padding-bottom:0rem!important;
    margin-bottom:0rem;
}
.sub-menu-content a {
  color: #0f0f0f;
  width:100%;
  padding:5px;
  margin-left:0rem;
  text-decoration: none;
  display: block;
  border-left:none;
}

.sub-menu-content a:hover {
    text-decoration:none;
    border-left:none;
    background-color: #00007a0d;
    color:#00007a!important;
    
}
    .header-menu {
        position: absolute;
        margin-top: 60px;
        width: 80%;
        height: 100vh;
        background: #e8e8e8;
        left: -100%;
        text-align: left;
        transition: 0.5s;
        margin-right: 0;
        padding: 0;
        box-shadow: rgb(18 18 18 / 8%) 4px 4px 12px 0;
        overflow:auto
    }
    .header-menu li:first-child {
        margin-top: 0;
    }
    .header-menu :last-child {
        padding-bottom: 0 !important;
    }

    .nav-item {
        border-top: none;
        margin-left: 0;
        box-sizing: border-box;
        border-bottom: 1px solid #b5b5b5;
        width: 100%;
        text-align: left;
        line-height: 60px;
        height: auto; /* 修改：自适应高度 */
        display: block;
    }
    .nav-link:hover,
    .sub-menu-link:hover {
        background: #00007a0d;
        transition-duration: 0.25s;
    }
    #check:checked ~ ul {
        left: 0;
    }
    
}
.menu-btn {
    position: relative;
    display: flex;
    justify-content: center;
    align-items: center;
    width: 22px;
    height: 60px;
    cursor: pointer;
    transition: 0.5s ease-in-out;
}
.menu-btn__burger,
.menu-btn__burger::after,
.menu-btn__burger::before {
    width: 22px;
    height: 2.5px;
    background: #00007a;
    border-radius: 3px;
    transition: 0.3s ease-in-out;
}
.menu-btn__burger::after,
.menu-btn__burger::before {
    content: "";
    position: absolute;
}
.menu-btn__burger::before {
    transform: translateY(-6.5px);
}
.menu-btn__burger::after {
    transform: translateY(6.5px);
}
.menu-btn.open .menu-btn__burger {
    background: 0 0;
    box-shadow: none;
}
.menu-btn.open .menu-btn__burger::before {
    transform: rotate(45deg);
}
.menu-btn.open .menu-btn__burger::after {
    transform: rotate(-45deg);
}


/* Show the dropdown menu (use JS to add this class to the .dropdown-content container when the user clicks on the dropdown button) */
.show {
    display:block;
}
.active {
    color:#00007a!important;
}
       @media(min-width: 1025px){
.sub-menu:hover .sub-menu-content {
  display: block;
  }
  }

JS #

/* When the user clicks on the button,
toggle between hiding and showing the dropdown content */
function subOpen(el) {
  const subMenu = el.parentNode.querySelector('.sub-menu-content');
  subMenu.classList.toggle("show");
}

// Close the dropdown menu if the user clicks outside of it
window.onclick = function(event) {
  if (!event.target.matches('.sub-menu-link') && !event.target.closest('.sub-menu-content')) {
    var dropdowns = document.getElementsByClassName("sub-menu-content");
    var i;
    for (i = 0; i < dropdowns.length; i++) {
      var openDropdown = dropdowns[i];
      if (openDropdown.classList.contains('show')) {
        openDropdown.classList.remove('show');
      }
    }
  }
}


//Mobile menu toggle code
const menuBtn=document.querySelector(".menu-btn");let menuOpen=!1;menuBtn.addEventListener("click",()=>{menuOpen?(menuBtn.classList.remove("open"),menuOpen=!1,document.documentElement.style.overflow="scroll",document.body.scroll="yes"):(menuBtn.classList.add("open"),menuOpen=!0,document.documentElement.style.overflow="hidden",document.body.scroll="no")})

内容的提问来源于stack exchange，提问作者writable755